Computer disk drives store information on magnetic disks. Typically, the information is stored on each disk in concentric tracks, divided into sectors. Information is written to and read from a disk by a transducer head, mounted on an actuator arm capable of moving the transducer head radially over the disk. Accordingly, the movement of the actuator arm allows the transducer head to access different tracks. The disk is rotated by a spindle motor at a high speed, allowing the transducer head to access different sectors on the disk. The transducer head may include separate or integrated read and write heads.
In a typical computer disk drive, the transducer head generally rides above the surface of the disk on a cushion of air that is created by the movement of the disk under the head. The distance of the head from the surface of the magnetic disk is known as the flying height of the transducer head. It is important to maintain the flying height of the transducer head within a desired range. For example, if the transducer head flies at too low a height, it is more likely to come into contact with the magnetic disk causing a loss of stored data. It is also important to ensure that the transducer head does not fly at too great a height. Where the transducer head is at too great a distance from the magnetic disk, the transducer head is said to be in a “high flying” condition. A “high fly write event” occurs when the transducer head suddenly is at too great a height from the disk to reliably perform write operations (i.e., suddenly enters a high flying condition).
Data written to a magnetic storage disk for storage while a transducer head is experiencing a high fly write condition may be lost. This is because the strength of the magnetic field generated by the write head decreases exponentially with distance. Accordingly, where the transducer is at too great a distance from the surface of the magnetic disk (e.g., during a high fly write event), the magnetic field produced may not be strong enough to induce the desired magnetic transitions in the storage disk. Therefore, it is important to detect high flying conditions in a computer disk drive, so that writing to the disk may be delayed until the transducer head has returned to a desired flying height above the surface of the disk.
Previous methods of detecting a high flying condition, including high fly write events, have included monitoring the amplitude of a signal produced in the read head when the read head passes over an automatic gain control (AGC) field on the disk. In general, automatic gain control fields are written to computer disk drive storage disks during manufacturing. AGC fields are usually located in “servo sectors” or “hard sectors,” which are areas extending radially across the disk that contain servo positioning information. In general, automatic gain control fields provide a reference magnetic field strength, so that the gain added to read signals by the hard disk drive's channel can be appropriately adjusted.
During the manufacture of a hard disk drive, servo sector information is written to the disk. During this “track writing” procedure, transducer heads having a write head that is about ⅔ the width of a data track are used. Because the AGC fields are the width of an entire data track, each AGC field must be written by at least three passes of the write head. This “stitching” together of the AGC fields causes the AGC fields to be vulnerable to manufacturing defects that result in AGC fields having unevenly spaced magnetic transitions. These unevenly spaced magnetic transitions can in turn result in magnetic fields of uneven strength. These manufacturing defects result in the production of signals in the read head that vary in amplitude. Although the varying magnetic strength of the AGC fields does not prevent them from functioning in connection with their primary gain control function, it does prevent high fly event detection systems dependent upon comparison with a standardized AGC field amplitude from operating with high sensitivity. In addition to defects within individual AGC fields, defects resulting in undesired variations in the magnetic strength of the AGC fields may arise between adjoining AGC fields, where the fields are aligned radially across all or a portion of the disk. Because of these variations between different AGC fields, they cannot be used to detect a change in the flying height of a transducer head from one AGC field to another. This is because a change in the detected amplitude from one AGC field to another could be caused by the above-mentioned manufacturing defects and variations, as well as by a change in the flying height of the transducer head.
Another method of detecting a high fly write event is disclosed by U.S. Pat. No. 5,909,330 to Carlson et al., assigned to the assignee of the present invention. According to Carlson, whether the flying height of a read/write head above a disk is within an acceptable range can be determined by the resolution of a signal read by the read head. In particular, this method relies upon the decrease in the number of detected signal peaks as the flying height of the read head increases.
Another method of detecting a high fly write event is disclosed by U.S. patent application Ser. No. 09/649,660 to Liikanen et al., assigned to the assignee of the present invention, and of which the present application is a continuation in part. According to Liikanen, the average amplitude of the signal derived from a plurality of servo sector position bursts or group of bursts is determined, and is compared to the observed amplitude of a one of the servo sector position bursts or group of bursts. If the comparison indicates that the strength of the observed signal is less than the average amplitude by at least a predetermined amount, a write fault error may be triggered. This method avoids the problems associated with monitoring a signal derived from the AGC fields. In particular, the servo sector position bursts provide a signal having a more consistent amplitude.
Sensitivity is important in detecting high fly write events, because such events are transient in nature. For example, a high fly write event may occur when a transducer head passes over a particle on the surface of the disk. The particle may cause a perturbation in the boundary layer of air supporting the transducer head, causing the head to fly at greater than a desired distance from the disk surface. In a typical high fly event, the transducer head flies at too great a height for one or two sectors of the disk. Therefore, it can be appreciated that detection of a high fly write event preferably is made on the basis of information collected over a single sector of the disk. Furthermore, because the change in height is small, and because the high fly write event may last for extremely short periods of time, it is important that the high fly write detection mechanism be extremely sensitive, so that high fly write events can be detected even at the beginning or end of such an event.
In addition, when a running average value is used as a reference, this method requires charging the algorithm used to develop the average value. Accordingly, such methods may not be suitable for use immediately following a head switch, because they are not able to immediately provide a reference value.
For the above stated reasons, it would be advantageous to provide a method and apparatus for quickly and reliably sensing a high fly write event in a computer disk drive. In addition, it would be advantageous to provide a method and an apparatus for detecting high fly write events that did not adversely affect the performance of the disk drive. Furthermore, it would be advantageous to provide a method and apparatus that can be implemented at an acceptable cost and that are reliable in operation.